As global oil reserves decline, the motivation to develop battery electric vehicles increases. As a consequence, improvements to the range and weight of electric vehicles become economically desirable.
To increase the efficiency of power conversion, the system voltage in battery powered electric vehicles (BEVs) has increased over the past decade from around 150V, to over 600V. Future systems of over 800V are expected. The safety risks of electrocution and fire inherent in the presence of such high voltages can be managed by methods such as chassis short circuit detection and automatic isolation, manually operated high voltage isolators, and insulating gloves worn by emergency services personnel, but risk management principles would suggest elimination of the problem is a safer solution than either personal protective equipment (PPE) or engineering solutions. Eliminating the problems caused by high voltage (HV) batteries can be achieved by lowering the system voltage. However, as above-mentioned, designers of BEVs are typically increasing system voltage, not decreasing it. Increasing the system voltage reduces conductor size and cost, and increases the efficiency, since the volt drop across IGBTs is lower at lower currents, reducing losses.
BEVs use one or more batteries with cells in series. Series cells require balancing circuits to ensure each cell has the same voltage. Typically, one cell balancer circuit is required across each cell. This means the use of very small cells leads to an expensive set of balancers. The function of the balancer circuit is well known to those skilled in the art and need not be detailed herein. Balancers are essential to reliably achieve the desired charge-discharge range and a long service life for the battery.
Even with cell charge balancers, the discharge depth of the cells does still vary with cell age, temperature and other factors, so eventually with all series strung batteries, a cell will fail, being forced into reverse polarity by the other cells, causing failure of the battery pack.
Cell bypassing can be provided to ensure cells do not over or under charge. However, only in very small cell batteries, can cell bypassing circuitry be built economically.
For BEVs, cell bypassing is uneconomic, and the whole battery has to be replaced after around eight years.
All series strung batteries require cell monitoring to detect cell under voltage and trip out the battery. Under voltage on any single cell therefore leads to battery failure and therefore in a BEV, vehicle failure. The reliability of the vehicle can only be improved by the use of two or more batteries.
It is desired to provide an electric motor that alleviates one or more difficulties of the prior art, or that at least provides a useful alternative.